Fentanyl is a commonly used narcotic that has wide applications in general anesthesia and critical care settings. We have previously shown that fentanyl, an opioid agonist, when administered to canines: (1) blocked b-adrenergic hemodynamic responses to epinephrine and isoproterenol; (2) did not effect the a-adrenergic effects of epinephrine or phenylephrine. These studies were extended to define the mechanism of fentanyl-catecholamine interactions by performing similar studies in canines with pertussis toxin, an inhibitor of G-protein signaling. These studies have implications for understanding the mechanism that accounts for altered hemodynamic responses due to fentanyl in anesthesia and in critically ill patients. The pilot portion of this protocol which used four dogs was completed in September 1999. Based on these results, the main portion of this protocol was begun. Twenty dogs will be enrolled in this phase of the study. All 20 will have completed parts one and two in phase two by June 16, 2000. For those dogs scheduled to have a third study, this protocol will be completed by September 16, 2000.